Magnetic demagnetizing devices



1962 G. BONNET ETAL 3,051,874

MAGNETIC DEMAGNETIZING DEVICES Filed Aug. 27, 1959 FIG. l. 9/

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INVENTORS 2 v GEORGES BONNET Q ATTORNEYS BY ROGER GARLOD United StatesPatent 3,051,874 MAGNETIC DEMAGNETIZING DEVICES Georges Bonnet and RogerGariod, Grenoble (Isere),

France, assignors to Commissariat a IEnergie Atomique, Paris, FranceFiled Aug. 27, 1959, Ser. No. 836,540 Claims priority, applicationFrance Aug. 30, 1958 3 Claims. (Cl. 317-157.5)

This invention relates to demagnetizing techniques of form-magneticsubstances.

Ferro-magnetic substances may, under certain well known conditions,present a so-called residual magnetization which must sometimes beremoved for scientific or industrial purposes. The demagnetization hasfor its object to render the intensity of magnetization as close aspossible to zero.

Magnetic demagnetization consists in producing in the sample undertreatment an alternating decreasing magnetic field. The followingconditions must be fulfilled:

(a) The decrease of the amplitude of the magnetic field must be regularand considerable. The initial amplitude must be at least four times thevalue of the coercive field; for laboratory uses the final amplitudemust be as weak as possible, in the presence of the coercive field. Ineffect, with regard to physical applications, the quality ofdemagnetization obtained increases with the ratio of the initial andfinal amplitudes.

(b) The mean value of the demagnetizing field must be nil to avoid anhysteretic magnetization phenomena, and the alternations of oppositesense must be identical, which implies perfect symmetry.

((3) Finally, it is advantageous in certain cases to allow the physicistto choose the law of decrease of the amplitudes of the curve of timevariation of the magnetic field.

The alternating magnetic field is generally produced by a coil energizedby an alternating current derived from an efiicient, constant source ofelectromotive force. In order to decrease the amplitude of the field,laboratories commonly make use of mechanical systems (for instance,magnetic circuits of variable reluctance, or liquid rheostats). Thesesystems are bulky, they lack flexibility, they are delicate to handleand generally they cannot be transported. Consequently they do not giverapid demagnetization or a fast rate of operation.

Automatic instruments based on the oscillating discharge of a condenserhave been introduced in the industry. Due to the weakness of the energyaccumulated, this principle does not result in sufficient physicaldemagnetization. In fact, the principle is used to determine the valueof residual magnetization of permanent magnets with respect toattraction, which implies a small variation of the magnetization.

The present invention relates to improvements in magneticdemagnetization devices which fulfil simply the condi tions mentionedabove.

The invention is essentially characterized in that the transmission ofalternating electric power, from the current source to the coil whichcreates the demagnetizing field, is effected through suitable electroniccircuits which affect the transmission, and which comprise, on the onehand, a power network which transmits in nearly its totality theelectric power, the alternations of opposite sense being transmitted ina perfectly symmetrical manner and, on the other hand, a compensatingnetwork which corrects the imperfections of the transmission, thesenetworks both connecting the secondary of an entry transformer, whoseprimary is connected to the current source, to the primary of an exittransformer whose secondary is connected to the coil which creates thedemagnetizing field.

A non-limiting embodiment of the improvements in magnetic demagnetizingdevices in accordance with the invention will be described hereafterwith reference to the accompanying schematical FIGURES l and 2. Theparticular features described with reference to this embodiment aredeemed to be part of the invention, it being understood that anyequivalent features may also be employed within the scope of theinvention.

FIGURE 1 is a diagram illustrating the principle on which are based theimprovements in accordance with the invention.

FIGURE 2 is a diagram of the electronic system of a demagnetizationdevice in accordance with the invention.

FIGURE 2a is a diagram of a modified electronic system of FIG. 2.

With reference to the diagram of FIGURE 1: the source of energy is analternating generator, not shown, of suitable frequency and power,connected to the primary winding of the entry transformer 1; the coil 2which creates the magnetic demagnetizing field may, in accordance withits impedance, be connected to different points of the winding of theexit transformer 3; the two half-windings 4 and 5 of the secondary oftransformer 1 are connected through an electronic network 6 to the twohalf-windings 7 and 8 of the primary of transformer 3. The network 6controls the transmission of electrical energy between the transformers1 and 3 and consequently the amplitude of magnetizing current of thecoil 2; when the transmission tends to zero at the end ofdemagnetization, an electronic network 9 which similarly connects asecondary winding 10 of the transformer 1 and a primary winding 11 ofthe transformer 3, makes it possible to reach a final stage, bycompensating the residual imperfections due to the wiring.

The advantages of the device are the following:

(a) The purely electrical regulation of power utilized makes possible avery high energy yield and minimum dimensions with respect to theinitial power required.

(b) The operation is entirely automatic and makes possible uses byremote control or with programming.

(0) The speed of operation only depends on the sample, which imposes alimit frequency on the alternating magnetic field, which makes itpossible to attain minimum demagnetization time.

(d) The network 6 makes it possible suitably to adapt thedemagnetization to any exterior phenomenon and to realize for theenvelope of the variation curve of the demagnetizing field, forms whichare judged adequate.

(e) The entirely symmetrical constitution of the device gives identityof alternations of opposite sense, these alternations being regulated bythe common network 6. The traces of dissyrnmetry which may remain aresuppressed by the addition of the compensating network 9.

(1) There is a very large ratio between the initial and final values ofthe demagnetizing field.

The device makes it possible to perform any desired action on theresidual magnetization of a ferro-Inagnetic body, ranging from fixingits value with respect to attraction, up to complete suppression, andfrom high laboratory performance to high power industrial applications.

Among the limitless number of possible embodiments of the transfer 6,that represented in FIGURE 2 is one of the simplest.

The characteristics of the currents flowing in the various windings areas follows.

Transformer 1:

Transformer 3:

Primaries 7 and 8 330 v. 300 ma. Primary 1-1 50 v. Secondary accordingto charge.

The transfer system generally indicated at 6 is constituted by a tube13, heating of whose filament 14 is efifected by the winding 12. Theswitch 15 makes it possible to suppress the heating current. The cathode16 is at the same potential as filament =14. The plate 17 is connectedto germanium diodes 18 and 19. During one alternation the current passesthrough the windings 4 and 7 and through diode 18, and during the otheralternation, through the windings and 8 and diode 19.

Decrease of the amplitude of demagnetization current is obtained asfollows.

The heating current of filament 14 is suppressed at the beginning ofdemagnetization by opening the switch 15. Cathode 16 cools and theinternal resistance of tube 13 increases. The -resulting decrease ofcurrent makes possible at the end of about 15 seconds a ratio of initialamplitude to final amplitude of the order of The compensating system 9comprises variable resistances and condensers Which compensate in phaseand amplitude the parasite connections between the various Windings. Theoptimum values depend on the material employed and on the construction.

In one specific embodiment of the invention the dimensions of theinstrument are 150 x 150 x 270 mm; the weight is 5 kilograms and theinitial power is 50 watts.

Finally, in the modified embodiment of the invention shown in FIGURE 2a,decrease of the current in the tube is obtained by adding a common grid20 to the tube 13, and regulating the potential of the grid by applyingthereto a variable negative charge, by any known means, here shown as avariable resistance 21 and a condenser 22 connected in the grid controlcircuit between switch and grid control 20.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. Magnetic demagnetizing device which provides in a simple mannerregular and considerable decrease of the amplitude of the demagnetizingfield, zero mean value of said field, and the possibility of choosingthe decrease low, said device comprising a source of alternatingcurrent, a

coil which creates a demagnetizing field, an entry transformer havingits primary winding connected to said current source, an exittransformer having its secondary winding connected to said coil, andelectronic circuits connected to the secondary Winding of said entrytransformer and connected to the primary winding of said exittransformer effecting and regulating the transmission of alternatingelectric power from said current source to said coil, the alternationsof opposite sense being transmitted in a perfectly symmetrical manner,said electronic circuits comprising a power network transmitting almostthe totality of the electric power and a compensating network producinga current component compensating for irregularities of the principalcurrent component produced by the power network due to residualdissimilarities of the power network, said power network comprising anelectronic tube having a filament, two diodes in two distinct andidentical circuits having in common only said tube, the alternations ofcurrent of opposite sense passing each through one of said two circuits,the current through said tube having a constant sense, and means forsuppressing the heating of the filament of said tube to thereby decreasethe amplitude of current traversing said tube.

2. Magnetic demagnetizing device according to claim 1, a grid for saidtube, and means to regulate the potential of said grid to therebydecrease the amplitude of current traversing said tube.

3. Magnetic demagnetizing device according to claim 1, wherein saidcompensating network comprises variable resistances and condenserscompensating in amplitude and in phase any parasite connections betweenthe several windings.

References Cited in the file of this patent UNITED STATES PATENTS2,207,392 Zuschlag July 9, 1940 2,256,117 Keeley Sept. 16, 19412,403,424 Zuschlag July 2, 1946 2,786,970 Connoy Mar. 26, 1957

